1. Field of the Invention
The present invention relates to Dielectric Barrier Discharge actuators and more particularly to improved Dielectric Barrier Discharge actuators for aerospace use.
2. Description of Related Art
All references listed in the appended list of references are hereby incorporated by reference, however, as to each of the above, to the extent that such information or statements incorporated by reference might be considered inconsistent with the patenting of this/these invention(s) such statements are expressly not to be considered as made by the applicant(s). The reference numbers in brackets below in the specification refer to the appended list of references.
Dielectric Barrier Discharge (DBD) actuators are surface-mounted, weakly ionized gas (plasma) devices consisting of pairs of electrodes separated by a dielectric and operated at high AC voltages as shown in FIG. 1a for a basic DBD and FIGS. 1b to 1d for new designs being disclosed herein. Devices, such as those shown in FIG. 1a, typically operate at frequencies in the range of a few Hz to tens of kHz, the optimum frequency being determined by the permittivity of the dielectric. An AC voltage, typically a few kHz and several kV applied across the dielectric, between the exposed and a buried electrode, generates a plasma on the surface of the dielectric. The plasma is accelerated by the field and imparts momentum to the airflow. A reaction force acts on the actuator in a direction opposite to the airflow. The electrically charged dielectric surface attracts charged ions in the air plasma, imparting momentum to the non-ionized air through many molecular collisions. Increasing the surface charge magnitude and/or increasing the ion density in the air plasma can increase momentum exchange to create an aerodynamic body force that accelerates neutral gas in the vicinity of the plasma for boundary layer flow control including separation control. One of the limitations of DBD performance is the lack of optimum dielectric materials, with current materials being ad hoc selections of readily available, high dielectric breakdown strength materials. Early devices were mostly made from thin, high dielectric strength materials such as Kapton® and the bulk of the work in the field focused on understanding the underlying plasma physics. More recently (last ten years), there has been an increase in interest in the devices for flow control in various applications and materials including glass, (PTFE) Teflon®, acrylic and ceramics such as alumina (Al2O3) have been used. Teflon®, with a dielectric constant of 2.1 and dielectric breakdown strength of 20 kV/mm is one of the top performing state-of-the-art materials. Actuators made of thick (several mm) Teflon® have been used to generate thrusts of the order 0.25 N/m of actuator at 25 kV rms [Ref. 1]. In all of the state-of-the-art materials, the force generated is seen to increase with an increase in the applied voltage, but above a certain threshold, which depends on the type and thickness of the dielectric material, the rate of increase is seen to decrease and then drop off Fine et al. [Ref 2] demonstrated that a titanium dioxide (TiO2) catalyst could be used to enhance the body force generated by Al2O3 actuators. More recently, Durscher and Roy [Ref. 3] have demonstrated that actuators made of silica aerogels form high performance but very brittle actuators.
The potential for this technology to enable new flight applications and significant improvements in flight vehicle concepts can be realized with materials designed for increased body forces that also provide higher force to weight ratios and improved robustness. Body force is equal to the product of the local electric field strength and net electric charge density in the ionized flow. The total force from a DBD device further depends on the total length (and thus area) of the device. Increasing the applied electric field, charge density and device length increases force, thereby enables a wider Reynolds number range of potential flight applications, from increased airfoil stall angles to improved jet engine turbine blade performance. The ideal actuator has high charge density, a dielectric material that supports high electric fields for charge acceleration and that is lightweight so the actuator can be applied over large areas while adding minimal weight. The actuator must also be of a low profile to enable easy installation without negatively affecting the airflow or requiring highly invasive modifications to the surface to which it is mounted. Furthermore, the dielectric material must be mechanically robust and chemically stable in order to be able to survive plasma over the surface, as well as harsh application environments, for extended periods. These include vibrations, high temperatures and contact with potentially damaging fluids and vapors such as those from jet fuel and hydraulic fluids in aviation applications.
The structure of DBD devices requires that electrodes be bonded onto the surfaces of the dielectric and therefore, the material should be amenable to this bonding for stable actuator performance.
It has been shown in the literature [Ref 4] that the ideal dielectric to increase force generation would have a low dielectric constant, near unity, (that of typical polymers ranges from 2-8 while those of bulk inorganic materials typically range from 4 and above) and a high breakdown strength (many kilovolts per millimeter) to enable ionization of the air. Additionally a low dielectric loss reduces heat generation in the dielectric, at the frequencies at which DBD actuators operate, and thus increases energy conversion efficiency and a catalytic layer to enhance the charge density in the air adjacent to the surface [Ref. 5].
It is a primary object of the present invention to provide an improved DBD actuator.
It is an object of the invention to provide an improved DBD actuator which has high charge density.
It is an object of the invention to provide an improved DBD actuator which has a dielectric material that supports high electric fields for charge acceleration.
It is an object of the invention to provide an improved DBD actuator that is lightweight so the actuator can be applied over large areas while adding minimal weight.
It is an object of the invention to provide an improved DBD actuator having a low profile to enable easy installation without negatively affecting the airflow or requiring highly invasive modifications to the surface to which it is mounted.
It is an object of the invention to provide an improved DBD actuator which has a dielectric material that is mechanically robust and chemically stable in order to be able to survive plasma over its surface.
It is an object of the invention to provide an improved DBD actuator functional in harsh application environments, such as vibrations, high temperatures and contact with potentially damaging fluids and vapors such as those from jet fuel and hydraulic fluids in aviation applications, for extended periods.
Finally, it is an object of the present invention to accomplish the foregoing objectives in a simple and cost effective manner.
The above and further objects, details and advantages of the invention will become apparent from the following detailed description, when read in conjunction with the accompanying drawings.